Epithelial-mesenchymal interactions are critical for the normal morphogenesis and maintenance of the crypt-villus axis. The molecules that regulate these reciprocal interactions have only recently begun to be described. We have identified a mesenchymal protein, epimorphin, as a putative regulator of epithelial morphogenesis. We show that this molecule has profound effects on morphogenesis and differentiation of the intestinal epithelium. However, the mechanism by which epimorphin exerts its effects is unknown, and the in vivo effects of inhibiting epimorphin expression in whole animals have not yet been described. Our hypothesis for this application is that epimorphin's effects are mediated, at least in part, by regulating the secretion o soluble substances that are important in morphogenesis. Based on our preliminary data, one such candidate may be sonic hedgehog (Shh)a critical regulator of gut-endodermal-mesenchymal signaling in early gut ontogeny. We show that blocking hedgehog signaling pathways by antibody infusion produces profound changes in postnatal gut morphology, with disorganized villi, runting and early death. These effects are recapitulated in vitro in an epithelial-myofibroblast co-culture model developed in our lab. The major hypotheses of the current proposal are: 1. Intestinal myofibroblasts produce epimorphin which regulates the formation and maintenance of the crypt-villus axis. 2. Hedgehog signaling pathways are required for the maintenance of the postnatal crypt-villus axis. The Specific Aims are: 1. Determine the mechanisms by which epimorphin induces crypt-villus morphogenesis and cytodifferentiation. 2. Determine the in vivo function of epimorphin by creating epimorphin -/- mice. 3. Determine whether the effects of hedgehog on postnatal epithelial morphogenesis and differentiation are mediated via epithelial-mesenchymal interactions, using an epithelial-myofibroblast co-culture system. The significance of this application is based on our recent identification of a mesenchymal/myofibroblast gene that plays an important role in the regulation of crypt-villus axis morphogenesis. A role for hedgehog signaling in postnatal gut ontogeny has also been established and will be further examined. Specific to the research scope for this RFA, these studies will characterize epithelial-mesenchymal cross talk underlying normal development. We will also establish a novel model, the epimorphin null mouse, for characterizing the molecular properties of the mesenchymal cell populations that regulate epithelial cell renewal in the developing and adult GI tract.